Fluor-amphibole glass-ceramics

ABSTRACT

A family of glass-ceramics containing a significant fraction of fluoramphibole crystals can be formed in the composition system (Li, Na)2O-(Ca, Mg)O-(B,A1)2O3-SiO2-F, through the crystallization in situ of glasses within this system. Fluorrichterite, fluor-magnesio-richterite and protoamphibole are readily-formed phases. Long, rod-shaped crystals have been observed by electron microscopy in certain of these materials which are thought to contribute to high mechanical strength and high dielectric breakdown strength.

United States Patent 1 Grossman 4 1 Oct. 1, 1974 FLUOR-AMPHIBOLEGLASS-CERAMICS [75] Inventor: David G. Grossman, Painted Post,

[73] Assignee: Corning Glass Works, Corning,

[22] Filed: Nov. 29, 1972 [21] Appl. No.: 310,374

[52] U.S. Cl 106/39.7, 106/39.6, 106/52,

106/54, 106/73.1 [51] Int. Cl. C03c 3/22,'CO3c 3/04, C030 3/30 [58]Field of Search 106/39.6, 39.7, 73.1

[56] References Cited UNITED STATES PATENTS 2,948,629 8/1960 Shelll06/73.l

3,054,685 9/1962 Shell 106/73.l 3,325,265 6/1967 Stookey 106/3963,360,333 12/1967 lvey 106/73.1 3,756,838 9/1973 Beall 106/39.6

OTHER PUBLICATIONS Takasagawa, N., et al., Crystallization isFlourrichterite Composition Glass, Chem Abs. Vol. 74, 1971 44939v.

Takasagawa, N. et al., Microstructure of Flourrichterite CompositionGlass-Ceramic l-laving High Mechanical Strength," Chem. Abs.

Primary Examiner-Winston A. Douglas Assistant Examiner-Mark BellAttorney, Agent, or Firml(ees van der Steere; Clarence R. Patty, Jr.

[5 7] ABSTRACT A family of glass-ceramics containing a significantfraction of fluoramphibole crystals can be formed in the compositionsystem (Li, Na) O-(Ca, Mg)0- (B,A1) O -SiO -F, through thecrystallization in situ of glasses within this system. Fluorrichterite,fluormagnesio-richterite and protoamphibole are readilyforrned phases.Long, rod-shaped crystals have been observed by electron microscopy incertain of these materials which are thought to contribute to highmechanical strength and high dielectric breakdown strength.

6 Claims, 2 Drawing Figures FLUOR-AMPHIBOLE GLASS-CERAMICS BACKGROUND OFTHE INVENTION Amphiboles are a family of silicate minerals which 5 occurin nature as fibers or fibrous masses. Varieties of amphibole andserpentine are used commerically in producing asbestos materials. Mostnotably, the variety of serpentine known as chrysotile g ,Si O (OH),,]is extensively used for this purpose. Amphibole asbestos ismoreresistant to chemical attack and is several hundred degrees morerefractory than the serpentine variety. However, amphibole fibers areless flexible than chrysotile and therefore less adaptable to commercialspinning. Amphibole fibers have been used in England for producingfire-protective clothing.

synthetically-formed amphibole materials, namely fluorine amphiboles,have been a topic for research since the l950s, after asbestos wasclassified as a strategic mineral during World War II. A comprehensivereview of the structure and synthesis of fluoramphiboles from melts isgiven by H. R. Shell, 1. E. Comeforo and W. Eitel in Synthetic AsbestosInvestigations, Bureau of Mines Report of Investigations, 5417 (1958).

The general structural formula of the fluoramphiboles may be given asW,,.,'X 'Y '(Z O,,) -F wherein the coordination of the cations to eitheroxygen or fluorine is: W 12, X 8, Y 6, and Z =4. W positions areoccupied by ions of radius 0.7 to 1.3A, chiefly Na, K, Ca, Mg and Li. Xpositions are occupied by ions of radius 0.7 to l.lA, including Ca, Na,Fe, Mg, Li and Mn. Y positions are occupied by ions of radius 0.5 to0.9A such as Mg, Fe, Mn, Fe, Al, Li and Ti. Z positions are occupied bysamll, high valence ions of four-fold coordination, principally Si butto a lesser extent (up to about 25%) Al.

The backbone of the amphibole structure is formed by double silicatechains which are crosslinked alternately by oxygen and fluorine. Eachdouble chain is made up of single chains arranged side by side in aherringbone pattern. The single chains are corsslinked alternately bythe X cations in eight-fold coordination and the Y cations in six-fold(octahedral) coordination.

A desirable feature of these fluor-amphibole crystals is their fibrousor needle-like habit. The growth of such crystals in situ in a glass ofappropriate composition could produce a fiber-containing glass matrixwherein the fibers would be undamaged and thus extremely strong. Thus, afiber-reinforced glass-ceramic article of high mechanical strength wouldbe provided.

A glass-ceramic article results from the controlled crystallization insitu of a glass article. Hence, the manufacturc of glass-ceramicsnormally involves three general steps: first, the compounding of aglass-forming batch containing a nucleating or crystallizationpromotingagent; second, the melting of the batch to form a homogeneous liquid andthe simultaneous cooling and shaping of the melt to form a glass articleof the desired dimensions and configuration; and, finally, the heattreatment of the glass article so produced in accordance with aspecifically defined time-temperature schedule to develop nuclei in theglass which act as sites for the growth of crystals as the heattreatment proceeds.

Since the crystallization in situ is brought about through anessentially simultaneous crystal growth on countless nuclei, thestructure of the glass-ceramic article comprises relativelyuniformly-sized crystals homogeneously dispersed in a residual glassymatrix, these crystals constituting the predominant proportion of thearticle. Thus, glass-ceramic articles are frequently described as beingat least 50% crystalline and in numerous instances are actually overcrystalline. In view of this very high crystallinity, the chemical andphysical properties of glass-ceramic articles are normally materiallydifferent from those of the original glass and are more closely relatedto those demonstrated by the crystal phase. Also, the residual glassymatrix will have a far different composition from that of the parentglass since the components making up the crystal phase will have beenprecipitated therefrom.

Because a glass-ceramic article is the result of the crystallization insitu of a glass article, conventional glass forming methods such asblowing, casting, drawing, pressing, rolling, spinning, etc. can usuallybe employed in securing the desired configuration to an article. Also,like glass, a glass-ceramic article is nonporous and free of voids.

US. Pat. No. 2,920,971, the basic patent in the field of glass-ceramics,provides an extensive study of the practical aspects and theoreticalconsiderations that must be understood in the manufacture of sucharticles, as well as a discussion of the crystallization mechanism.Reference is made thereto for further explanation of these matters.

SUMMARY OF THE INVENTION I have now discovered that glass-ceramicarticles consisting essentially of fluor-amphibole crystals dispersed ina residual glassy matrix may be produced from clear to slightly opalglasses over a particularly defined composition area consistingessentially, in weight percent as calculated from the batch, of about48-75% SiO- 527% MgO, 1-13% MgF 0-15% A1 0 0-l0% B 0 and 3-2()% total ofone or more oxides selected in the indicated proportion from the groupconsisting of 0-15% CaO, 4l6% Na O, and 310% Li O.

In fluor-amphibole crystals produced from these compositions, the Wposition may be unoccupied but is preferably occupied by Na* or Li ions,the X positions are occupied by Ca or Mg, the Y positions are occupiedby Mg, B, or Al, and the Z positions are principally occupied by Si butin some instances by Al. X-ray diffraction examination has shown threedistinct types of fluor-amphibole crystals present in the crystallizedglass: fluorrichterite structures centered around the formula Na CaMg SO F fluor-magnesiorichterite structures centered around the formula NaMg Si O- F and lithium-containing protoarnphibole structures centeredaround the formula M LiMg6,5Si8O F Fluor-magnesio-richterite is a synthetic fluor-amphibole reported by G. V. Gibbs, J. L.

Miller and H. R. Shell in Synthetic F luor-Magnesio- (NaCaitMg Alsi-Oz Fland fluoreckermaniteXNay I Mg4AlSi O 2F2) types are crystallized fromglass compositions stoichiometric to these compounds.

Numerous substitutions of other ions into the described crystals may bemade with varying results. Thus, K may be substituted for Na, while Zn,Cd, Sr, Ba and Pb may be substituted for Ca. Also, Te, Sn and Ti may besubstituted for Si, while Fe", Mn, Ni, Cu, Co, Zn, Cr, P, Sb and V maybe substituted for Mg. In general, however, no special benefit isderived from such substitutions.

While some of the compositions within the scope of the inventioncrystallize to a rather blocky microstructure comprising crystals of lowaspect ratio, electron micrograph studies reveal the presence ofunusually long, rod-shaped or fiberlike crystals in certain other ofthese compositions which are thought to contribute to high mechanicalstrength. Many of the amphiboles are also characterized by highdielectric breakdown strength in an electric field, a property usefulfor high voltage applications.

The production of fluor-amphibole glass-ceramics according to the methodof the present invention typically comprises melting a batch for a glassconsisting essentially, in weight percent, of about 48-75% SiO 5-27%MgO, 4-13% MgF O-l5% A1 0-l0% B 0 and a total of 3-20% of at least oneoxide selected in the indicated proportions from the group consisting of(ll5% CaO, 446% Na O, and 3-10% Li O, simultaneously cooling the melt atleast below the transformation range thereof and shaping a glass articletherefrom, and thereafter heating the glass article to a temperature inthe range from about 750l,OOOC. for a period of time sufficient toobtain the desired fluoramphibole crystallization in situ in thearticles The product of this method is a glass-ceramic articlecontaining a major proportion (at least about 50% by volume) of thedescribed fluor-amphibole crystals, although in certain cases minorcrystal phases of differing composition and structure may also bepresent.

The batch for the article may be made up of any constituents, whetheroxides or other compounds, which, upon melting to form a glass, will beconverted to a composition within the aforementioned range. For example,the required presence of fluorine in the glassceramic articles of theinvention is expressed and computed for purposes of convenience in termsof a quantity of MgF whereas in actuality MgF may or may not comprise abatch material and fluorine may be incorporated into the batch using anyof the well-known fluoride compounds employed for this purpose in theglass art.

The invention may be further understood by reference to the followingdetailed description thereof, and to the appended Drawing, wherein FIGS.1 and 2 are electron photomicrographs showing the crystallinemicrostructure of two fluor-amphibole containing glassceramic articlesproduced according to the present invention.

DETAILED DESCRIPTION or THE INVENTION H As previously set forth, batchmaterials for the glassceramic compositions of the present invention maycomprise any of the compounds known and used in the art for meltingglasses. Examples of suitable materials include pure sand, magnesiumoxide, magnesium fluoride, pure alumina, anhydrous B 0 sodium carbonateand lithium carbonate. Preferably the batches are ball milled to insurehomogeneity, and are then melted in pots, crucibles, tanks or the like,typically at temperatures in the range of about 1,400l,500C. The meltsare then formed into glass articles and annealed in the 550-600C.temperature range, although where commercially desirable, annealing maybe omitted and the formed glass articles immediately subjected to acrystallizing heat treatment. Soda-containing compositions within thescope of the invention typically produce very clear and stable glasses,whereas glasses high in fluorine may be cloudy in appearance.Lithia-containing glasses are generally opalescent, except in the caseof major lithia additions or where alumina is present. Fluorineretention in these glasses averages -90% of fluorine additions in thecase of covered crucible melts.

The incorporation of nucleating agents into the glasses of the inventionto promote and control crystallization during heat treatment is notrequired. Electron micrograph studies suggest that a phase separationwhich occurs as the glasses are heated to the crystallization range isresponsible for the nucleation of the fluor-amphibole crystal phases inthese systems. Evidence of crystal growth appears after heating theglass at temperatures as low as about 600C, and complete crystallizationis normally obtained after heating at temperatures near l,O00C. for 4hours.

Generally, heat treatments in the temperature range from about 750-l,OOO C. for times in the range from about 2-24 hours are employed. Thecrystallization process is both time and temperature dependent so that,at lower temperatures, longer treatment times will be required and viceversa. The use of a nucleating step or soaking periods at intermediatetemperatures within the described range is not required, althoughheating rates to maximum temperatures should be limited in order toaviod deformation of the article. A treatment which comprises a hold ofat least about 4 hours in the 900l,OOOC. temperature range is preferredwhere maximum crystallization of the article is desired.

Table I below lists a number of examples of glass compositions which maybe thermally crystallized to fluor-amphibole glass-ceramics according tothe present invention. Also listed are the molar compositions of theglasses and the fluor-amphibole crystals most closely related to themolar composition of each glass. Thermal crystallization of theseglasses does not in all instances produce the fluor-amphibole phases towhich the glasses are most closely related by composition, as willhereinafter more fully appear.

'T'ABLE'I SiO 58.4 49.9 59.5 60. MgO l9.6 19.2 15.0 15.3 MgF 7.6 7.4 7.77.9 CaO 6.8 6.7 Na O 7.5 7.4 l l.5 ll 7 TABLE I Continued Li O A1 6.3 BQ, 4.4

g 9.5 Molar I Composition Na CaMg Si O F Na- CaMg Ti Si-,O- F- Na;,MgAlSi O F Na Mg BSi O F Related Fluor- I fluoreckermanite Amph|bo1eCrystal fluoriclsiterite fluorichterite fluoreckermanite (boron analog)7 8 810-, 50.2 51.2 65.0 64.6 MgO 19.2 19.6 18.2 17.0 MgF 7.4 7.6 5.67.3 CaO 13.4 13.6 Na O 3.7 3.8 11.2 11.1 1.i- .O A1 0,, 6.1 B 0 4.2 TiOglolar omposition NaCa Mg AISi O H NaCa Mg BSi O F Na,M Si Q F Na M Si OF. Related Fluorfluoredenite lluorfnagnes i ofliior indgngsiza-Amph|bo1e Crystal fluoregenite (boron analog) richterite richterite I 1011 12 $10; 63.9 62.4 64.2 65.5 MgO 18.2 20.6 19.0 19.8 MgF 5 6 5.7 5.65.7 CaO 11.2 Na O 12.3 11.3 9.0 Li O A1 0 :1

2 Molar Composition u gs im gm z 4 gB.8 ||.4 -'lll.-1 2 4 gm um am 2 128m i2 ai 2 RelatedFluorfluor-magnesiofluor-magnesiofluor-magnesiofluor-magnesio- Amph|bo1eCrystal richterite richterite richterite richterite 13 14 15 16 S10,62.1 63.3 52.6 49.0 MgO 26.0 15.9 17.7 12.3 MgF, 8.1 8.2 6.8 6.4 CnO 6.15.7 N320 Li O 3.9 5.9 A1 0; 6.7 10.4 2 :1

B 4 16.8 Fe O 16.3 Molar I Composition Li Mg si o- F Li=,Mg,AlSi,.O F.BaCaMg;,Si,,O ,F CaMg,Fe Al Si, O ,;F-= Related Fluor- Amphibole Crystalproto-amphibole proto-amphibole fluortremolite fluorhornblende The heave"66115156511655" illustrate that fluoramphibole glass-ceramics may beprepared over a rather wide range of composition in the (Li, Na) O- (Ca,Mg)O-(B, Al) O -SiO- -F system. The exact fluoramphibole crystal phasepresent in each material depends in part on the composition of the baseglass. Thus fluorichterite (Na CaMg Si O F and fluoredenite (NaCa MgAlSi O F- phases may be produced from glasses within the above-describedcomposition area which contain 3-l5% CaQ, 4-l6% Na O. a total of 7-20%Ca0 Na. ,O, and no lithia, while fluormagnesio-richterite (Na- Mg Si O Fand fluoreckermanite (Na-,Mg AISi O- F phases are expected uponcontrolled crystallization of glasses within the abovedescribedcomposition area containing 4-16% Na- O, no lithia and no lime.

Precise identification of the desired crystal phases in all cases may beextremely difficult. For example, it is hard to distinguishfluorrichteritc. fluorcckermanitc and fluoredenite crystals by X-raydiffraction analysis of these materials because 01 the closesimilarities in the unit cell dimensions of these crystals. Thus inmany 1. The presence of minor secondary phases appearing in combinationwith the major fluor-amphibole phases is reported where identificationhas been made. Strength values are modulus of rupture determinations inpounds per square inch of cross-sectional area as measured on abradedbar samples. All of the glassceramic articles shown in Table 11 werecrystallized according to a heat treatment schedule comprising heatingto 800C. at about 200C. per hour, holding at 800C. for four hours,heating to 1,000C. at 200C./hour, holding at l,()0()C. for 4 hours, andfinally cooling to room temperature.

TABLE ll Major Fluorfluorrichterite fluorrichteritefluor-magnesiofluor-magnesio- Amphibole Phase (Na CaMg Si O- F (Na CaMgSi O F richterrte nchterrte ZMg SigOQZFg) (Na Mg Si O F Secondary PhasesVisual Description Modulus of Rupture Major Fluor- Amphibole PhaseSecondary Phases Visual Description Modulus of Rupture Major Fluor-Amphibole Phase Secondary Phases Visual Description Modulus of Rupture(psi) Major Fluor- Amphibole Phase Secondary Phases Visual DescriptionModulus of Rupture none fine-grained fracture; blocky microstructurenone fine-grained fracture; small. needle-like none fine-grainedfracture; small needle-like crystals crystals 21.200 18.200 16.600

5 6 7 8 fluorrichterite fluorrichterite fluor-magnesiofluor-magnesio-(Na CaMg Si O F (Na CaMg Si O F richterite richterite -z gssin az z) zgs u u z) none none none none some deformation; coarse-grained somedeformation; coarse-grained fine-grained fracture; fiber-like crystalsfine-grained fracture;-

fiber-like crystals fracture fracture 9 l l l l 2 fluor-magnesiofluor-magnesiofluor-magnesiofluor-magnesiorichteriterichterite richterite riehterite z gs u zz z) -z gs n z z z) z ge n zzz) z gs n z-z zl none none none none fine-grained fracture; fiber-likecrystals fine-grained fracture; fiber-like crystals fine-grainedfracture; fiber-like crystals fine-grained fracture; fiber-like crystalsl3 l4 l l6 proto-amphibole proto-amphibole fluor-tremolitefluor-tremolite (LiMg Si O F (LiMg Si O F (Ca Mg Si O F (Ca Mg Si O- Flithium silicate beta-spodumene eristobaiite magnetite;lithium-magnesium cristobalite silicate fine-grained fracture;fiber-like crystals coarse-grained fracture; some deformationfine-grained fracture (psi) From the data presented in Table ll it canbe seen that crystals of the fluorriehterite type (Na CaMgfisixoggF-g)are readily formed from soda-and limecontaining compositions even in thepresence of Bog and/or A1 0 Thus, attempts to produce major fluoredenite(NaCa- Mg AlSi O F crystal phases or boron analogs thereof (NaCa Mg BSiO F generally produce fluorrichterite bodies, although the presence ofminor fluoredenite phases cannot be ruled out in view of the similarX-ray diffraction patterns of fluorrichterite and fluoredenite.

Generally, fluorrichterite bodies are characterized by somewhat blockycrystalline microstructure and good strength. The substitution of minoramounts of acceptable ions for Na, Ca, Mg, Si, or F in the crystalstructure of these materials typically does not alter the crystalmicrostructure. Problems of deformation and coarse grain size may beencountered during the crystallization of articles in the fluoredenitecomposition area. Therefore compositions consisting essentially, inweight percent as calculated from the batch, of about 48-75% SiO 4-l3%MgF- 5-27% MgO, 3-1 5% CaO, 4-1 6% Na O and 7-20% total of CaO and Na Oare preferred in the production of fluorrichteritc glass-ceramics.

major proportion of boron and aluminum in these compositions remains inthe residual glass rather than being incorporated into the crystalphase. Glass-ceramic articles of this type are characterized byneedle-like crystalline microstructure and good body strength.

Compositions in the Na O-MgO-SiO -F system which are related to crystalsof the fluor-magnesio-richterite type (Na Mg,,Si,,O F generally produceglassceramics having the most fiberlike crystalline microstructure. Thecrystals formed in this system are almost invariably offluor-magnesio-richterite composition, and the glass-ceramic articlesproduced are highly crystallized and of good strength. Minor amounts ofsecondary crystal phases such as tridymite and eristobaiite may beformed during crystallization of these glassceramics, depending upon thecomposition of the base glass.

The fiberlike microstructure of these materials is evident in FIGS. 1and 2 of the drawing, which are electron photomicrographs of thefracture surfaces of glassceramic articles of compositions 7 and 8,respectively, shown in the above tables. The white bars in themicrographs represent one micron. The differences in crystallinemicrostructure between FIGS. 1 and 2 are attributed to the difference influorine level between the compositions. The higher fluorine content ofcomposition 8 results in more extensive nucleation and a finer fibrousmicrostructure than is seen in composition 7.

Compositions which are particularly preferred in the manufacture offluor-magnesio-richterite glassceramics consist essentially, in weightpercent as calculated from the batch, of about 48-75% SiO 4-l3% MgF5-27% MgO, O-l5% Al O and 4-16% Na O.

Compositions in the Li O-MgO-SiO -F and Li O- MgO-Al O -SiO -F systemsare useful in the production of glass-ceramics wherein proto-amphibole(LiMg 5Si O F crystals constitute the principal crystal phase. Othercrystal phases formed in minor quantities in these systems includetridymite, Li- MgSiO Li- Si- O and spodumene. The lithium-containingfluoramphibole glass-ceramics are particularly valuable from thestandpoint of dielectric breakdown strength, with several compositionsgiving values in the range of about 3,000-4,000 volts/mil. Glasscompositions particularly suited for use in the preparation of theseproto-amphibole glass-ceramics consist essentially, in weight percent ascalculated from the batch, of about 48-75% SiO 15% A1 0 4-l 3% MgF 5-27%MgO and 310% Li O. Generally, boron-for-aluminum substitutions in thesesystems produce glasses which are more difficult to form and whichcrystallize to coarsergrained glass-ceramics than the alumina-containingcompositions.

The production of alkali-free fluor-amphibole glassceramics of thefluortremolite (Ca Mg,-,Si O F type typically involves difficulties inthe area of glass quality, with many compositions being difficult tomelt or difficult to form without devitrification. Some deformation onceramming and coarse microstructure in the finished product are alsoencountered. As shown by Composition 16 in Table ll, fluortremolitecrystals may also be obtained from alkali-free glass compositions moreclosely related to fluor-hornblende than to the fluoramphiboles.Generally, the presence of some glassstabilizing agent such as BaO, A1 0TiO SnO Fe O or ZnO is required to obtain good glass-ceramics in thesealkali-free systems.

Physical properties other than those shown in Table II were determinedon certain of the compositions within the scope of the presentinvention. The article shown as Example 7 of Table II demonstrates aYoungs Modulus of about 14.5 X 10, a Shear Modulus of 6.0 X 10, andPoissons Ratio of 0.20. This article has a Knoop hardness of 570, anaverage coefficient of thermal expansion over the range from roomtemperature to 800C. of about 97 X l0' /C., and a thermal conductivityof 0.00370 cal-cm/cm--sec-C. The article is also characterized by adielectric breakdown strength of 3,000 volts per mil. The acid andalkali durabilities of these compositions are generally good. Thephysical properties of the other compositions shown in Tables I and IIare not expected to differ significantly from the values shown above forExample 7.

From the foregoing description and examples it is apparent that thefluor-amphibole glass-ceramics of the present invention provide physicaland electrical properties desirable for a wide variety of electronic andtechnical applications.

I claim:

1. A glass-ceramic article consisting essentially, in weight percent onthe oxide basis as calculated from the batch, of about 48-75% SiO 527%MgO, 4-l3% MgF 0l5% A1 0 0-l0% B 0 and 320% total of alkali metal oxidesselected in the indicated proportion from the group consisting of 4-l6%Na O and 3-l0% Li O, a major proportion of the volume of the articlebeing comprised of fluor-amphibole crystal phases selected. from thegroup consisting of fluor-magnesiorichterite, proto-amphibole, andfluoreckermanite.

2. A glass-ceramic article according to claim 1 which is essentiallyfree of lithia and which contains 416% Na O by weight, a majorproportion of the volume of the article being composed offluor-amphibole crystal phasesselected from the group consisting offluormagnesio-richterite and fluoreckermanite.

3. A glass-ceramic article according to claim 2 which is essentiallyfree of A1 0 and B 0 the major propor-- I tion of the volume of thearticle being composed of fluor-magnesio-richterite crystals.

4. A glass-ceramic article according to claim 1 which is essentiallyfree of soda and which contains 3-l0% Li O by weight, a major proportionof the volume of the article being composed of protoamphibole crystals.

being composed of fluortremolite crystals.

1. A GLASS-CERAMIC ARTICLE CONSISTING ESSENTIALLY, IN WEIGHT PERCENT ONTHE OXIDE BASIS AS CALCULATED FROM THE BATCH, OF ABOUT 48-75% SIO2,5-27% MGO, 4-13% MGF2, 0-15% AL2O3, 0-10% B2O3, AND 3-20% TOTAL OFALKALINE METAL OXIDES SELECTED IN THE INDICATED PROPORTION FROM THEGROUP CONSISTING OF 4-16% NA2O AND 3-10% LI2O, A MAJOR PROPORTION OF THEVOLUME OF THE ARTICLE BEING COMPRISED OF FLOUR-AMPHIBOLE CRYSTAL PHASESSELECTED FROM THE GROUP CONSISTING OF FLUOMAGNESIO-RICHTERITE,PROTO-AMPHIBOLE, AND FLUORECKERMANITE.
 2. A glass-ceramic articleaccording to claim 1 which is essentially free of lithia and whichcontains 4-16% Na2O by weight, a major proportion of the volume of thearticle being composed of fluor-amphibole crystal phases selected fromthe group consisting of fluor-magnesio-richterite and fluoreckermanite.3. A glass-ceramic article according to claim 2 which is essentiallyfree of Al2O3 and B2O3, the major proportion of the volume of thearticle being composed of fluor-magnesio-richterite crystals.
 4. Aglass-ceramic article according to claim 1 which is essentially free ofsoda and which contains 3-10% Li2O by weight, a major proportion of thevolume of the article being composed of protoamphibole crystals.
 5. Aglass-ceramic article according to claim 4 which is essentially free ofB2O3.
 6. An alkali-free glass-ceramic article consisting essentially, inweight percent on the oxide basis as calculated from the batch, of48-75% SiO2, 5-27% MgO, 4-13% MgF2, 0-15% Al2O3, 0-10% B2O3 and 3-15%CaO, a major proportion of the volume of the article being composed offluortremolite crystals.